Page:The Journal of Indian Botany.djvu/355

Rh striking changes in the aspect and content of the vegetation from season to season, and may be called seasonal succession. As a result of the three well defined climatic seasons, rainy, cold, and hot, there are three equally well marked vegetational seasons. The traveller across the Upper Gangetic Plain is impressed with the numerous groves of stately trees. These groves are without exception planted, and are composed of species that either are not native to the immediate area or that cannot survive in the young stages without protection. Some of these trees would quite readily perpetuate -themselves were it not for the pressure of the human factors.

Aside from cultivated fields and the groves of planted trees, the aspect of most of the area is determined by herbaceous plants. Here and there are tracts dominated by shrubs and small trees, but across these the herbaceous vegetation extends unchanged. Obviously there is no change in the content of the woody vegetation in response to change in season, but there is response in the matter of leaf fall and time of blooming. Even these changes are relatively inconspicuous in comparison with the changing aspect of the herbaceous and under-shrub vegetation. It is periodicity in the small and very abundant annual and perennial plants that renders the fact of seasonal succession so conspicuous.

During the rainy season, when all conditions are favorable for maximum plant growth, the whole country is covered with a luxurient mantle of herbaceous vegetation. Much of it is annual. The perennial herbs and undershrubs attain their maximum development at this time. The vegetation approaches hygrophytic, with large soft thin leaves and tender stems. It is during the rainy season that the tropical aspect of the flora as well as its composition appears most clearly.

With decrease of rain, and the lower temperature of the cold season, much of the rank growth of the rainy season disappears. What survives is gradually eaten down by grazing animals. The hygrophytic annuals are replaced by mesophytic annuals, especially Compositae, of more temperate connections. Perennials lose their luxurient tropical appearance, and in every way the vegetation is less in amount, number of species, and conspicuousness.

With the advent of the hot season, the high temperature and extreme aridity complete the destruction of the annual vegetation that is not situated in favored edaphic spots, and little more than the persistent perennials remain. There are a few annuals, but the number is small. The perennials now present a very different appearance from that of the rainy season, and even the cold season. All but the youngest leaves fall, and the more deli-